While an association between many steroid receptors and the 90 kilodalton heat shock protein (hsp90) has been recognized for several years, the function of this interaction and the mechanism of establishing this interaction are unresolved. Recent advances have been made in elucidating the mechanism of hsp90 binding to chick progesterone receptor. From earlier studies on hsp90 and chick progesterone receptor (PR), the following hypothesis for assembly of hsp90-PR complexes is presented: a hetero-oligomer consisting of hsp90, hsp70-- another major heat shock protein-- and a 60 kilodalton protein (60) must first bind receptor, followed by ATP-dependent dissociation of hsp70 and p60 from the newly formed hsp90-PR complex. This hypothesis essentially describes a molecular chaperone mechanism in which hsp70 and p60 are required for proper presentation of hsp90 to PR. The goal of this proposal is to understand the assembly of hsp90-PR complexes; in pursuit of this goal, the assembly hypothesis presented above will be tested. The least understood component in the hypothesis is p60, so early specific aims will focus on this protein in preparation for developing a fully defined hsp90-PR assembly system in vitro. Specific Aims: 1. Prepare antibody and molecular probes for cloning p60 CDNA. 2. Purify p60 and characterize its structure. 3. Characterize the interactions of p60 with hsp90 and hsp70. 4. Measure the dynamics of hsp90 turnover in PR complexes. 5. Develop a fully defined system for assembly of hsp90-PR complexes. This study will make significant contributions toward a basic understanding of the cellular management and processing of steroid receptors. In addition, binding of hsp90 to PR might provide a valuable model system for understanding general assembly mechanisms for multi- protein complexes.